Shop Chevrolet Impala Accelerator Pedals

You may have a bad IAC ( idle air control) but try cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.

Does your scan tool have data stream capabilities? If so, you should graph APP 1 volts, APP2 volts, TPS 1 volts, TPS 2 volts, actual engine rpm, and desired engine rpm. See what those pids are doing when your problem is occurring. Maybe that information can paint a better picture.

Probably would also be a good idea to scan for codes on the manufacture side. Sometimes manufacture codes don't show up on the OBD2 side. You need a enhanced scan or the factory scan tool to get more information.

My scan tool has the ability to view live data.

I also have tuning software that has all the features of consult II. (However I can't get it to open after upgrading to Windows 10)

I guess that gives away the car is not stock...

This is where I usually lose peoples help or go on a wild rabbit chases.

Given its been fine up until now I figured it had to be a simple component failure unrelated to the modifications given all had been completely fine up until now.

Here is a condensed list of mods. Any specifics feel free to ask if your still with me. Haha!

Vortech supercharger
Walbro drop in pump (255lph)
Injector Dynamics 725cc fuel injectors.
UpRev GT MAF (recalibrated OEM)
Full exhaust (headers , cats and exhaust)
Jim Wolf technologies clutch and fly (dealer installed)
UpRev Osiris tuning software (unlocks and re-flashes oem ecu)
Very conservative dyno tune from a world renowned tuner.

Viewing live data I don't see anything crazy. Ignition advance follows RPM, MAF is not erratic, coolant temp, IAT all steady and nothing high or low.

I can't view APP only TPS and It seems to correlate. It's a gradual rise in RPM; maybe 1k per 1.5 - 2 seconds so it's not a huge increase.

I wish my software worked. I can log no problem.

I see no fluctuations with vac and I have no boost leaks

"Given its been fine up until now I figured it had to be a simple component failure unrelated to the modifications given all had been completely fine up until now"
you are discounting that a modified component has failed- They are the most LIKELY to fail.


getting the scanner working is key, I wonder if the throttle plate is actually moving OR something causing hi RPM,
look at the throttle control motor - is the circuit ECM to motor actuator , open, shorted to power, look at trouble trees p2100 and up as that is the fly be wire system wire colors ECM pinouts, tests etc....


GTG first susprct is always aftermarket

I see what your saying, I just have a feeling its not the blower, fuel system or exhaust causing the issue.

I'm not even in boost when it ramps up.

I have wide bands and I don't see any fluctuation or any lean or rich conditions from target AFR's.

I'm un-installing and re-installing the software now.

Just like with any drive-ability problem, the first thing is to identify what is causing the concern. You need to start here first before guessing and swapping things. Collecting data from suspected systems, doing visual inspections, and interpreting data to figure out what is going on is important.

Once you identify the what, then proceed to figure out why. Once you isolate the why and repair it, then figure out why the why occurred, so it doesn't happen again.

Sometimes you can get caught up in rabbit holes diagnosing stuff. That is where thinking outside the box is helpful. If you see things in data or reactions by something that don't make any sense, sometimes resets can fix the issue. I've run into a few vehicles that doing a reset by disconnecting the battery, touching the terminals together, and turning on the ignition for a period of time, actually corrected the anomalies. Somewhere in software or logic functions the PCM gets caught up in loops and does strange things. Why this happens I don't have a clue. Fully powering down the PCM, just like your PC, can bring things back into order sometimes. Recognizing the weird things that are unusual come with experience. Sometimes connecting a certain brand of scan tool or device to the OBD2 connector or even accessing certain modules can cause all kind of weird things to happen.

Our shop uses an Autel scanner and the other day I connected into 06 Hyundai Santa Fe. When I did a full system scan the engine shut off. I noticed the engine shut off when the scanner was getting codes from the immobilizer system. I didn't condemn the vehicle. I suspected it had something to do with how the scanner communicated with the module. It was repeatable of course. Just so many variables to deal with on these vehicles and it gets worse with modifications.

sorry I had to go... What I was thinking was


Because the ECM sees no conflict between APP and TPS I question if the throttle plate is really moving. and something else is causing RPM.


The RPM ranging up may require several incidents to set codes they are NOT 1 failure type.
I think your pulling up on throttle is causing a out of range issue and the ECM defaults - the issue causing high RPM is no longer present, the one time pulling on APP assy would not code and you return to 'normal' .


have you looked at MAF - is it gong up as RPM goes up?
With a blower positive manifold pressure how do you handle EVAP purge, and similar functions?

This is where I usually lose peoples help or go on a wild rabbit chases.

That is perfectly understandable. Most of the professionals in the auto repair industry don't get too many modern modified (electrically and mechanically) vehicles in the shop or even want to deal with them. Modifications open up another world of possible problems. Once you start getting into software changes and other mods, the factory repair information becomes useless.

I know that in factory software, the PCM is monitoring the correlation between sensors to be sure they are in agreement. If the PCM sees an increase in idle speed or rpm, but doesn't see APP voltage changes, it should go into reduced power mode and either fix the throttle position or kill injectors. It should also flag codes. If it sees sensor correlation errors or sensor circuit issues it should do the same thing. There has to be redundancy for safety. It is how the factory software is written. When software is altered or modified, it is possible those parameters for TAC issues are not included or altered so they don't perform the same way? Hard to say without knowing exactly how the modified software is written.

If it was factory software and you had no error codes or reduced power messages, I'd say yes, the PCM is seeing an APP1 and 2 change for some reason and bringing up engine speed. Then you need to find out what is causing the voltage changes for APP inputs. Really don't know why pulling on the accelerator pedal causes the engine to come back down, unless you're changing the APP voltage input voltages into something the PCM software doesn't usually see or recognize.

If you had a 4 channel lab scope you could also scope the TPS and APP sensors and actually see if the throttle is opening and causing the higher idle or if there is something else causing the engine to idle up. Really need some data from those sensors. Probably would also be interesting to see what the injector pulse width is doing if you don't see changes in TPS voltages or throttle percentage during the idle up concern. You also have to know what those voltages should be normally, of course. Need a baseline to work from.

my first thoughts....
Is your 'scanner' up to the job? these codes will likely be so called manufacturers specific.
Also will scanner allow you to view 'live' data - ?
important- vehicle is STOCK ?
I will look at TSB's and reflashes awaiting answers...

Update on the eagle. Still no luck.

Got a diagnostic tool on it finally, showing no code.

Gas pedal mechanics and everything seems fine down there, reason behind it sticking still unknown.

Running out of ideas, we had a guy from transmission shop stop in and look at it and he didn't have any idea either.

Next step will be to take the whole tranny out and check it, even though the pan seemed clean.

Any thoughts anyone? thought diagnostic tool might catch it if it was a sensor, but that was no luck...

Thanks in advance for any help

Hold on a second. Is throttle still way too high now? For now forget trans as this began with stuck throttle then didn't shift into gear later. You should be able to see the throttle move with a helper pushing the pedal at the the intake throttle body. All this with everything off. Look at the plate to see it shut completely visually. Can you move it by hand more toward lower idle right at the engine?

Is the shifter moving all the way to the trans? A trans tech should have noticed that but who knows? Instinct tells me both issues have a common issue but could be wrong. IDK but if the idle is wildly way too high now - still - then perhaps there's a fail-safe not to engage trans?

Again, I just think this is something out of whack underhood that both throttle and shifter share. That struggle to slow down really shouldn't have been any harder on trans than going up a steep hill IMO but again who knows for sure?

I'm not ready to tear trans apart with this situation and not so sure of you getting that suggestion but I'm not there inspecting this in person either,

T

checked the throttle- lowered the idle and it seemed to make the engine a bit happier but still not engaging trans.

Friend's cousin was over and said he had the exact same problem a few weeks ago and it had to do with the neutral safety switch, he replaced his and it fixed it no problem.

So that is what we are trying now. Only $13 for one so if thats it not I have no problem. But if that doesn't work I'm out of ideas.

Will be putting it in tomorrow.

For now, friend finally got his Porsche 944 running :D

Neutral safety switch is so it won't START in anything but N or P. Nothing to do with whether is engage a gear to move IMO. To save you the $13 buck those if basic (probably) you could just jump with a paper clip type thing and it's no longer in the picture.

Does PARK hold the car? Then car off does it release when put to N or any if not running? IDK - this may have just snapped something from the trauma,

T

Well we got the wrong thing sent to us anyways. So no on the neutral safety switch?

Hmm.. no idea then.

Park holds the car fine. Only will move in neutral.

Fairly certain it isn't mechanical as I think I would've heard something snap/grind/etc.

Any other ideas?

Oh- I think I forgot to mention this.. there is an electrical buzzing sound of some sort from what we think is the tranny or very near. unplugged another something and the buzzing stops but car will not start . will try to get pictures of what was unplugged to make it stop.

Update.

Guy from transmission shop says its bad, replacing or rebuilding. Didn't elaborate though. Whatever, only paid him twenty bucks. Useless. I'm still convinced its electrical, but other friend thinks torque converter.

Tearing it out soon, seems I'm gonna have problems finding an exact match for it. Closest I've seen is a 96 Mirage that has the exact same engine. For $800.
Not really interested in spending that much on a refurbished transmission hat might or not might work.

Theres Mirages all the way up to 01 with the same engine- would it be likely that those might work? Or would we have electrical/computer problems?

If not, seems we're shit out of luck because its rediculously hard to find parts for this transmission anyways.

Maybe better off trying to find an engine swap that wouldn't be too difficult so I have something with easier to obtain parts.

So, the transmission specialist told you what was wrong but you think he is wrong and you know better?

You can't find a transmission for the price you want to pay so you think changing the engine is the answer?

Maybe you just need to look for a new car.